Blood, rare cells and vital tissues are generally stored at very low temperatures, namely at about −80 to −196° C. In particular, bone marrow cells, hematopoietic stem cells and like rare cells are effectively used in the treatment of intractable diseases such as leukemia, and a technology of storing them for a long period of time has been demanded. For the storage at such very low temperatures, blood, rare cells and vital tissues are generally stored in tightly closed containers therefor immersed mainly in liquid nitrogen.
As for the cryopreservation containers for such storage, vials made of polypropylene, for instance, are commercially available for use on the laboratory level; they are inexpensive and convenient from the handling viewpoint.
With the recent development of cord blood banks, bag-like containers excellent in low-temperature resistance and flexibility have been proposed. For example, containers made of a laminated film composed of a polyimide film and fluorinated ethylene-propylene polymer film (Patent Document 1) and containers made of a tetrafluoroethylene-ethylene copolymer film (Patent Document 2) have been proposed. Further, Patent Document 3 discloses cryopreservation containers molded from an electron beam-irradiated, biaxially stretched ethylene-vinyl acetate copolymer film. Patent Document 4 discloses cryopreservation containers molded from a biaxially stretched, crosslinked polyethylene film.
On the other hand, the present inventors developed cryopreservation containers which are constituted of a laminated film consisting of an ultrahigh molecular weight polyethylene layer and a thermoplastic resin layer compatible with the ultrahigh molecular weight polyethylene layer and are superior to the cryopreservation containers disclosed in Patent Documents 1 to 4, and applied for a patent for the same (Patent Document 5).
When blood, rare cells and vital tissues are preserved in the cryopreservation containers mentioned above in liquid nitrogen, the cryopreservation containers are further packaged so that the contents may be protected in case of damage of the containers and liquid nitrogen may be prevented from entering the cryopreservation containers. For packaging them, packages made of a perfluoroethylene-propene copolymer, among others, are generally used.
However, such packages are fluororesin-made ones and therefore the molding or processing for manufacturing them must be carried out at elevated temperatures. In spite of this, their heat seal strength is not so high but the sealed portions are possibly in danger of peeling away. After packaging of a cryopreservation container in a package, the mouth portion of the package is heat sealed for tight closure. This mouth portion is also susceptible to peeling and there is the possibility that liquid nitrogen may enter the inside. This work is carried out in medical institutions such as cord blood banks and heat sealing under severe conditions is a burden on workers. Further, when polypropylene vials for cryopreservation or cryopreservation containers described in Patent Documents 1 to 4 are packaged, there is the possibility that when the packages are damaged and liquid nitrogen enters the inside, the cryopreservation containers may also be damaged.
Ultrahigh molecular weight polyethylene films are formed by cutting processing and are relatively thick. When they are used as materials for packages, the conduction of heat to the contents may possibly be affected.
Known as materials for packages for cryopreservation containers are polytetrafluoroethylene, polychlorotrifluoroethylene, tetrafluoroethylene/hexafluoropropylene copolymers, tetrafluoroethylene/ethylene copolymers and polyimides, among others (cf. e.g. Patent Document 6 and Patent Document 7). However, those fluororesins which are used in preparing the conventional packages require elevated temperatures for sealing and their sealability is insufficient in some cases.    Patent Document 1: Japanese Patent Publication S49-008079    Patent Document 2: Utility Model Publication S55-055069    Patent Document 3: Japanese Patent Publication S55-044977    Patent Document 4: Japanese Patent Publication S62-057351    Patent Document 5: Japanese Kokai Publication H08-173505    Patent Document 6: Japanese Kokai Publication H11-139459    Patent Document 7: Japanese Kokai Publication 2003-267471